This invention relates to the formation of trench, or groove patterns in semiconductor surfaces for the purpose of dielectric isolation. Separation or isolation of the semiconductor elements by the use of trenches filled with a dielectric material allows for increased density of the elements.
The current trench technology encompasses several trench filling and planarization techniques. One well-known method involves the steps of: etching a substrate to form the trenches; depositing the dielectric in the trenches which results in forming an excess dielectric layer on the upper surface of the adjacent substrate; and, finally, removing the excess layer by mechanically polishing the smoothing the dielectric layer back to the upper surface of the substrate at the level of the top of the filled trenches. This method involves the preliminary step, before the trench patterns are etched, of depositing a hard layer of material which is more resistant to mechanical polishing than is the subsequently deposited dielectric material. In the polishing process, the greater resistance to the polishing indicates to the polishing entity that the upper surface of the substrate, and therefore the top of the trenches, has been reached and that polishing should cease. At the same time, however, the hard resistant layer introduces greater stress on the substrate and, together with the mechanical polishing vibration, can weaken and degrade the device. Another drawback to this method, of utilizing the gross movement of a mechanical polisher, is the difficulty in achieving the accuracy desired for a semiconductor device.
A common alternative method currently in use involves depositing and then chemically etching the dielectric or polysilicon layer. The dielectric, or polysilicon, is deposited in the grooves or trenches until they are filled. In the process of filling the trench, a thick excess dielectric or polysilicon coating is necessarily deposited on the upper surface of the substrate between the trenches. The excess dielectric or polysilicon is then removed, and planarity reestablished, by a reactive ion etch back to the original substrate surface. Again, the method is relatively inexact and difficult to control.
Doping and selective etching have been employed in several recently developed methods of planarization of filled trenches in patterned semiconductors. U.S. Pat. No. 3,892,608 issued to Kuhn discloses a planarization method whereby a thin doped oxide layer is preliminarily deposited onto the grooved semiconductor substrate using a spin-on technique. Excess doped oxide found on the upper surface of the substrate is removed using standard photolithographic techniques, leaving the doped oxide in the grooves. An undoped polysilicon layer is then deposited over the entire surface and the substrate heated in order to drive the doping impurities from the doped oxide into the undoped polysilicon in the trenches. A selective etch is then used to remove the undoped polysilicon remaining on the upper surface of the substrate.
Similarly, U.S. Pat. No. 4,391,033 issued to Shinozaki, makes use of an upward diffusion of a doping impurity from a previously deposited layer and the subsequent use of a selective etchant. Shinozaki's doping of the polysilicon layer results from the utilization of not only the upward diffusion but also simultaneous diffusion in from the top surface of the layer, whereby, the doping process is stopped when the impurities meet at the "center" of the polysilicon layer. Shinozaki presumes a conformally filled trench having the geometric uniformity necessary for its concurrent doping methods. Such exactness in deposition is impractical under batch conditions and is, in fact, extremely difficult to achieve unless extremely thin fills are used. Alternatively, Shinozaki teaches ion implantation and thermal activation of the implanted boron impurities.
Each of the above-referenced patents involves the deposition of additional layers introducing additional time and cost factors along with the obvious drawbacks of greater opportunity for errors, and inexactness in both the deposition and the doping steps, and greater stress on the substrate due to repeated deposition and heating steps.